Non-linear dielectric effect in the isotropic phase of ferroelectric liquid crystals

We use a phenomenological model to find theoretically the non-linear dielectric effect (NDE) in the isotropic phase above the isotropic to chiral smectic-C (I-SmC^*) phase transition. We find an analytical expression for the NDE in the isotropic phase above the I-SmC^* phase transition. The temperature dependence of the NDE is presented in the isotropic phase of the I-SmC^* phase transition.     

Self-organization of N<Superscript>*</Superscript> inclusions in SmC<Superscript>*</Superscript> free-standing films

The behaviour of freely suspended smectic-C* ( SmC^*) films at the bulk SmC^*-cholesteric ( N^*) phase transition has been investigated using polarized-reflected-light microscopy. Our experimental observations show that above the bulk SmC^*- N^* phase transition the N^* order appears in different ways according to the film thickness. In thin films, the conventional layer-by-layer thinning occurs. In films of intermediate thickness N^* inclusions nucleate inside the SmC^* film. The distortions of the in-plane orientational order of the SmC^* host phase induce elastic interactions between the inclusions and lead to their self-organization in chain-like structures. Both the dynamic of the chaining and the parameters driving the equilibrium distance between the inclusions in the chain are investigated. In thick films, N^* fingers grow inside the film. The influence of the experimental conditions on the various processes is analysed. Received 1 July 2002 / Published online: 15 April 2003 RID="a" ID="a"e-mail: Philippe.cluzeau@univ-lille1.fr     

Electro-optical Kerr effect in the isotropic phase of the two antiferroelectric liquid crystal mixtures

We have experimentally studied for the first time the electro-optical Kerr effect and the pre-transitional behavior in the isotropic phase of two antiferroelectric liquid crystal mixtures, W-232 and W-204D, composed of rod-like ester molecules exhibiting the direct smectic-A to isotropic (SmA-I) phase transition. The Kerr law has been confirmed for the two compounds and the variation of inverse Kerr constant with temperature above the smectic–isotropic transition temperatures were determined. Both the mixtures with very broad antiferroelectric phase around room temperature have similar sequence of the phases (i.e., Cr-SmC*A-SmC*-SmA-I). Although, the pre-transitional behavior is usually complex in the isotropic phase of the chiral smectic liquid crystal compounds, the investigated compounds showed a similar behavior compared to that of nematic–isotropic behavior.     

Energetics of 2D colloids in free-standing smectic-C films

The formation of regular colloid patterns in free-standing smectic films at the transition from the smectic-C to the isotropic or nematic phase is well known experimentally. The self-organization of isotropic or nematic droplets is caused by their mutual interaction, mediated by elastic distortions of the local director in the surrounding liquid crystal. These distortions are related to the anchoring conditions of the director at the droplet border. We describe analytically the energetics of the liquid crystal environment of a single droplet in one-constant approximation. A method of complex analysis, Conformal Mapping, is employed. Following a suggestion of Dolganov et al. (Phys. Rev. E. 73, 041706 (2006)), energetics of chain and grid patterns built from the colloids are investigated numerically in order to explain experimentally observed formations and their director fields.     

Isotropic to ferroelectric and antiferroelectric liquid crystals phase transitions

This review is an attempt towards a unified picture of the direct transitions from the isotropic liquid to ferroelectric and antiferroelectric liquid crystalline phases formed by rod-like and bent-core molecules. The Landau–Ginzburg theories of the phase transitions between the isotropic liquid to ferroelectric and antiferroelectric liquid crystalline phases in compounds composed of chiral rod-like molecules and achiral bent-core molecules are presented. This includes a discussion of the nature of the order parameters and the nature of the various types of phase transitions. The various predictions are compared with the available experimental results.     

Competition between ferroelectric and flexoelectric polarization in freely suspended smectic films

We report a detailed ellipsometric study of freely suspended films of chiral liquid-crystal compounds possessing smectic-A and smectic-C phases. In the temperature region between the smectic-A - smectic-C bulk and surface transitions, a discontinuous reconstruction of the tilt profile across the film is observed in the presence of a constant d.c. electric field. Comparison of the measured ellipsometric quantities with values calculated from model tilt profiles reveals a competition between a structure possessing a homogeneous tilt direction and large ferroelectric polarization and a structure with opposite tilt direction in the two film halfs and large flexoelectric polarization. Received 21 October 1998     

Dynamic mechanism of the ferroelectric to antiferroelectric phase transition in chiral smectic liquid crystals

The temperature-induced phase transition between the chiral smectic phases, antiferroelectric (smectic-C(A)*) and ferroelectric (smectic-C*), is found to occur through solitary wave propagation. We measure the free energy, which shows a double well shape in the entire SmC(A)* temperature range and the global minimum is found to shift from the antiferroelectric order to the ferroelectric order at the transition temperature. However, any significant supercooling is not observed and the transition cannot be described by the first order Landau-de Gennes theory, where the double well potential exists only in a narrow range of temperatures. This implies that the SmC(A)*-SmC* transition can occur only nonhomogeneously through the solitary wave propagation which overcomes the high energy barrier between the two minima.     

Confinement-induced orientational order in a ferroelectric liquid crystal containing dispersed aerosils

The study of the smectic-A to chiral smectic-C(*) phase transition of the liquid crystal S-(+)-[4-(2(')-methyl butyl) phenyl 4(')-n-octylbiphenyl-4-carboxylate] (CE8) containing dispersed hydrophilic aerosils reveals novel properties, important to understanding quenched disorder and confinement in ferroelectric liquid crystals. Smectic layer compression leads to a distribution of transition temperatures inducing smearing of the macroscopic data across the transition. A pronounced confinement-induced pretransitional tilted order is observed.     

Novel twist grain boundary smectic-C phases

Several new kinds of smectic-C twist grain boundary phases (TGBC) have been observed during the last few years. These pure compounds or mixtures exhibit unusual textures with polygonal lattices (square or hexagonal grids) in the plane normal to the TGB helix. The structure of these new phases seems to be complex and different from reported and predicted TGBC phases. In this article, we review the main results obtained on these different new phases, and we propose new TGBC structures based on the well-known splayed polarization --twisted director structures adopted by chiral smectic-C's in planar aligned (bookshelf) cells. The observed square or hexagonal lattices are made of superimposed pairs of unwinding lines due to the suppression of the helix within smectic-C blocks by the grain boundaries (unwinding walls). A lattice-free TGBC occurs if the helix within smectic-C blocks is suppressed completely. Received 15 March 2001     

Free energy of semiflexible polymers and structure of interfaces

The free energy of semiflexible polymers is calculated as a functional of the compositional scalar order parameter and the orientational order parameter of second-rank tensor S_ij on the basis of a microscopic model of wormlike chains with variable segment lengths. We use a density functional theory and a gradient expansion to evaluate the entropic part of the free energy, which is given in a power series of .The interaction term of the free energy is derived with a random phase approximation. For the rigid rod limit, the nematic-isotropic transition point is given by , N and w being the degree of polymerization and the anisotropic interaction parameter, respectively, and the degree of ordering at the transition point is 0.33448. We also find that the contour length of polymer chains becomes larger in a nematic phase than in an isotropic phase. Interface profiles are obtained numerically for some typical cases. In the neighborhood of isotropic-isotropic interfaces, polymer chains tend to align parallel to the interface on the polymer-rich side and perpendicular on the poor side. When an isotropic region and a nematic region coexist, orientational order parallel to the interface is preferred in the nematic region. Received: 28 May 1998 / Revised: 12 August 1998 / Accepted: 8 September 1998     

Molecular structure and physical parameters of some ferroelectric liquid crystal systems

We investigate thermodynamic properties and spontaneous polarization of ferroelectric liquid crystal (SmC^*LC) systems: smectic C matrix + chiral adding (ChA). The fact of existence of an optimum concentration of ChA, for which the studied SmC^*LC systems have the maximum value of spontaneous polarization, and a certain concentration, at which the phase transition of the chiral tilted smectic C phase to the smectic A phase occurs, is established. Temperature dependence curves for the free energy, configuration energy, specific heat, and the orientational order parameter are calculated in the temperature range of existence of liquid crystal mesophases. The curves are in good agreement with the experimentally observed trends of the SmC^*LC properties.     

Field-induced phase transitions and reversible field-induced inversion of chirality in tilted smectic phases of bent-core mesogens

Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle ≈ 45^°). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high electric fields ( 25-35V μm^-1) an antiferroelectric electro-optical response based on the collective rotation of the molecules around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric switching and of the field-induced inversion of the chirality is discussed on the base of XRD, ¹³C- and ¹H-NMR investigations, dielectric and electro-optical measurements.     

A self-consistent approach for the SmA–SmC* phase transition

By using an analogy with the Maier–Saupe theory of the nematic phase, a mean field theory of the chiral smectic-C (SmC*) phase is constructed. As an order parameter of the SmC* phase, the tilt angle θ is selected, and the feedback effect of θ is introduced into the hindered rotational potential about the molecular long axis. By solving the self-consistent equation for the order parameter θ, the second-order phase transition appears. Also, to describe the SmA–SmC* phase transition under an electric field, a coupling term between a molecular dipole moment and an external electric field is introduced to the potential function. The electroclinic effect in the SmA phase and the hysteresis effect in the SmC* phase, which were described phenomenologically in the past, are found directly from the microscopic theory.     

Disordering to order: de Vries behavior from a Landau theory for smectic phases

We show that Landau theory for the isotropic (I), nematic (N), smectic-A, and smectic-C phases generically, but not ubiquitously, implies "de Vries" behavior: i.e., a continuous A-C transition can occur with little layer contraction while the birefringence increases significantly once the system moves into the C phase. Our theory shows that 1st order A-C transitions are also possible. These transitions can be de Vries-like, but in general need not be. Generally, de Vries behavior occurs in models with unusually small orientational order and is preceded by a first order I-A transition. These results correspond well with experimental work to date.     

Magnetic properties of a multi-domain model for smectic-C liquid crystals

The magnetic properties of a multi-domain smectic-C phase liquid crystal are calculated as a function of tilt angle and magnetic field orientation. Numerical results are presented for the overall diamagnetic susceptibility and magnetic torque density. It is predicted that a first-order reorientation phase transition can occur for the case of a free smectic-C whose tilt angle exceeds 35° at some temperature. These calculations can be used for critically evaluating the multi-domain model and for studies of the temperature dependence of the smectic-C tilt angle.     

Statics and kinetics at the nematic-isotropic interface in porous media

We extend the random anisotropy nematic spin model to study nematic-isotropic transitions in porous media. A complete phase diagram is obtained. In the limit of relative low randomness the existence of a triple point is predicted. For relatively large randomness we have found a depression in temperature at the transition, together with a first order transition which ends at a tricritical point, beyond which the transition becomes continuous. We use this model to investigate the motion of the nematic-isotropic interface. We assume the system to be isothermal and initially quenched into the metastable régime of the isotropic phase. Using an appropriate form of the free energy density we obtain the domain wall solutions of the time-dependent Ginzburg-Landau equation. We find that including a random field leads to smaller velocity of the interface and to larger interface width. Received 12 November 1998 and Received in final form 15 March 1999     

Blue quantum fog: chiral condensation in quantum helimagnets

It is shown that a condensation transition involving a chiral order parameter can occur in itinerant helimagnets, in analogy to the transition between the isotropic phase and the phase known as blue fog or blue phase III in cholesteric liquid crystals. It is proposed that such a transition is the explanation for recent neutron scattering results in MnSi. Predictions are made that will allow for experimental tests of this proposal.     

Viscoelastic modes in chiral liquid crystals

Viscoelastic properties of liquid crystals are very important for applications like display technology. However, there are not many direct techniques to study them. In this review, we describe our studies on the viscoelastic modes of some chiral liquid crystals using dynamic light scattering. We discuss viscoelastic modes corresponding to the C director fluctuations in the chiral smectic C phase and the behaviour of the Goldstone-mode near the chiral smectic C-smectic A phase transition. In cholesteric liquid crystals, we consider the director fluctuations in a wavevector range comparable to the inverse pitch of the cholesteric. Here, the study of the scattered light in the vicinity of the Bragg reflection using a novel geometry will be presented.